CMP is an important technique in manufacturing semiconductor devices. Semiconductor integrated circuit chips are manufactured by forming conductive layers, insulating layers, or other thin film layers in a prescribed order on a wafer, patterning the layers depending on need by photolithography and etching, and cutting and separating each chip on the wafer after forming all layers. When there are convexities and concavities or steps in the substrate when a film is formed, the thickness of the film formed on the substrate will be thin in parts and step coverage will worsen, causing lower yield and other problems. Also, problems occur in that focus during exposure is not fixed and precise patterns cannot be transferred because convexities and concavities appear in upper layers due to the effect of the lower patterned layers. For this reason, the surface of the film material must be planarized, and CMP is used for such a purpose.
In CMP, efforts are made to increase throughput (the number of wafers that can be polished in a unit of time) by polishing at the maximum possible speed. However, when tungsten (W), copper (Cu), titanium nitride (TiN) or another metal film is polished at high speed, there is a problem in that erosion increases. Therefore, in metal-based CMP, high-speed polishing is performed first, and at the point when the thickness of the metal film reaches around several tens of nanometers, by switching from high-speed polishing to low-speed polishing. As a result, erosion is reduced.
A method is known in which an eddy current sensor is used as one method for measuring the thickness of a metal film during high-speed polishing (Japanese Laid-open Patent Publication No. 2004-525521). The eddy current sensor measures the thickness of a metallic film by using a high-frequency magnetic field. Therefore, the metal film is polished at high speed while the film thickness is measured using the eddy current sensor, high-speed polishing is terminated when the eddy current sensor reaches a prescribed threshold value, and a switch is made to low-speed polishing.
However, when the thickness of the polishing pad changes due to polishing pad wear, the remaining metal film will also have a thickness that corresponds to the variations in the polishing pad thickness. When the polishing pad thickness fluctuates due to polishing pad wear, the remaining metal film will also have a thickness that corresponds to the fluctuations in the polishing pad thickness. Since the distance to the metal film being polished increases when the polishing pad is thick, the thickness of the metal film becomes greater than the target thickness, even when high-speed polishing has ended at the point when the eddy current sensor has reached a prescribed output value. Also, since the distance to the metal film being polished is reduced when the polishing pad is thin, the thickness of the metal film is less than the target thickness, even when high-speed polishing has ended at the point when the eddy current sensor has reached a prescribed output value. Variations in the remaining metal film lead to variations in the polishing time in low-speed polishing (barrier clear polishing) that follows, and the remaining film also is thick when the polishing pad is thick. Therefore, there is a problem in that CMP throughput is reduced. When the polishing pad is thick, the remaining film will also be thick. Since the remaining film becomes thin when the polishing pad is thin, quality degradation is liable to occur due to erosion and the like.